This invention relates to a low-temperature sintered dielectric ceramic body compound of fine barium titanate grains and a small amount of cadmium silicate flux, the ceramic body having a high dielectric constant and a smooth temperature coefficient of dielectric constant, especially suitable for use in high quality multilayer ceramic capacitors.
Multilayer ceramic capacitors having a smooth X7R dielectric temperature coefficient comprise a substantial portion of the large ceramic capacitor market. Progress toward better X7R ceramic bodies having higher dielectric constants (K) has been slow. It is a well known contrariness rule that whatever is done to increase the dielectric constant, also degrades the smoothness of the dielectric temperature coefficient, and vise versa. However, a most striking advance that violates that rule has resulted from the discovery that capacitors with ceramic bodies of very fine grains especially around 0.8 micron diameter, tend to have both a higher dielectric constant (K) and at the same time smoother temperature coefficient of capacitance (TCC) compared to large grained (1 micron and larger) ceramic bodies of the same composition.
In multilayer capacitors the fine grain structure has the further advantage that the space between adjacent layers of buried electrodes may be commensurately smaller leading to higher capacitance per unit volume, or lower cost per microfarad. Fine-grained bodies are made by sintering very fine particle (e.g. 0.2 micron) powders having a high surface energy and advantageously sinter at a lower temperature. But for such fine-grained bodies the same old contrariness rule applies. For example, fine-grained bodies of pure barium titanate hot pressed and sintered have K of about 3700 and are far from meeting the X7R specification. To meet this X7R standard the K over the operating temperature range of from -55.degree. C. to 125.degree. C. varies no more than 15% relative to the K at room temperature. But, when sintered with 11 wt % of a sintering film (CdBi.sub.2 Nb.sub.2 O.sub.9) at 1130.degree. C., the ceramic is densified and easily meets the X7R standard but has a room temperature K of only 1500. Alternatively the addition of 15 wt % of a bismuth titanate flux (2Bi.sub.2 O.sub.3.3TiO.sub.2) and sintering at 1110.degree. C. to achieve full densification yields a K of 500 at room temperature and meets the X7R standard. Using 21/2 wt % of an additive composed of NbO.sub.2.5.CoO with the pure fine grained barium titanate provided an X7R ceramic body with a K of 3000 but required a sintering temperature of 1300.degree. C. These results are reported by Hennings et al in the paper entitled Temperature-Stable Dielectric Based on Chemically Inhomogeneous BaTiO.sup.3, Journal of the American Ceramics Society, Vol. 67, No. 4, 1984, pages 249-254.
Another well known contrariness rule, exemplified by these examples, is that the addition of a sintering flux to the ceramic start mixture reduces the sintering temperature necessary to achieve densification, and thereby reduces the amount of precious metals needed to prevent the buried electrodes from melting during sintering. The addition of flux also tends to make the temperature coefficient smoother. However, such additions of flux tend to drastically reduce the dielectric constant of the sintered ceramic. This role applies without exception to all dielectric ceramic materials regardless of ceramic grain size.
The capability for being sintered to maturity and high density at low firing temperatures makes it possible to include electrodes of silver and low precious metal content (e.g. Pd) buried in the ceramic during firing. Cost is consequently reduced by the lower temperature in manufacturing and more significantly by the much less expensive electrode materials.
A third contrariness rule is that the addition of a sintering aid or flux melts at sintering providing a medium in which the process of simultaneous dissolution and recrystallization of the ceramic ensues, simultaneously promoting grain growth and thus exacerbating the problem of providing a high dielectric constant and a low TCC. In fact this rule was not at all contrary before only a few years ago when large grains were perceived as a goal toward achieving high dielectric constant ceramics. For example, see U.S. Pat. No. 4,898,844 (Feb. 6, 1990); U.S. Pat. No. 4,266,265 (May 5, 1981); U.S. Pat. No. 4,120,677 (Oct. 17, 1978); U.S. Pat. No. 4,066,426 (Jan. 3, 1978); and U.S. Pat. No. 3,885,941 (May 27, 1975). In the patent U.S. Pat. No. 3,231,799 (Jan. 25, 1966) there is disclosed the use of separate compounds of niobium and tantalum in the start materials to provide a high K body with a smooth temperature coefficient of capacitance (TCC). These and other elements have been recognized as grain growth inhibitors, e.g. as in the above-noted U.S. Pat. No. 4,120,677 (Oct. 17, 1978) and U.S. Pat. No. 4,898,844 (Feb. 6, 1990). The above-mentioned patents with issue dates in parentheses and the patent application are assigned to the same assignee as is the present invention.
Mechanical comminution of ceramic powder particles to reduce particle size becomes ineffective at lower than about 1 micron and particle shapes are jagged and non-spherical. Only recently have methods for making finer powder become known. For example see U.S. Pat. No. 4,654,075 (Mar. 31, 1987), such fine particles made by either the hydrothermal or the emulsion processes are advantageously spherical, have a relatively narrow particle size distribution and have an average particle size that may be varied upward from 0.1 micron in the process of manufacture. Such particles have a very high energy density and therefore will sinter at a lower temperature than larger particles of the same composition. They also are more reactive at sintering with other compounds in a ceramic start material.
It is an object of this invention to provide a ceramic powder mixture for use in manufacturing a high dielectric constant ceramic body sintered at a the low temperature of 1100.degree. C., having grains of average size less than 0.9 micron and having a high dielectric constant and a smooth X7R temperature coefficient of the dielectric constant.